1. Field of the Invention
The present invention is related to a multiple input multiple output (MIMO) antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system. More particularly, the present invention is related to an MIMO antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system that are capable of adjusting a transmission power and a transmission mode of a signal.
2. Descriptions of the Related Art
As mobile communication systems have evolved from the third generation (3G) to later generations, such as the beyond third generation or the fourth generation, limited spectrum resources have hindered the development of the wireless communication technology. To increase the data rate and more efficiently utilize the spectrum, some technologies and schemes have been used to improve the spectrum utilization factor, such as optimization modulation schemes, program code multiplexing systems or MIMO technologies. Over recent years, MIMO technology has been widely applied in the industry. For example, the emerging Worldwide Interoperability for Microwave Access (WiMAX) standard and the new generation of Wireless Local Area Network (WLAN) are both incorporated with the MIMO technology.
MIMO systems indicate that signals are transmitted and received through multiple antennas synchronously. The MIMO system adopts a plurality of antennas both at the transmitting end and the receiving end, so that data can be transmitted via a plurality of signal transmission channels to increase the data rate. More specifically, a signal is divided into multiple portions at the transmitting end for synchronous transmission through a plurality of antennas. Because individual portions of the signal are transmitted through different signal transmission channels, they may arrive at the receiving end at different times. To prevent failure of recombination due to the different received times of individual portions, the receiving end has a plurality of antennas to receive these signals simultaneously. By digital signal processing and re-computation, the separate signal portions are then recombined into the original signal rapidly and properly.
By dividing the signal, the traffic in the single signal transmission channel can be decreased so that the signal transmission distance may be increased. Accordingly, MIMO technology has been able to speed up the signal transmission and avoid the use of additional spectrum, as well as increase the signal transmission distance. Therefore, many wireless network apparatuses have adopted MIMO technology to meet the increased requirements of signal transmission speed and distance. Thus, MIMO technology has become a key technology that must be adopted in the new generation of mobile communication systems.
Compared to a single-antenna system, an MIMO antenna system is capable of receiving a larger data amount and consequently, has a higher data rate. In the MIMO antenna system, a signal transmission channel may be established between each of the transmitting antennas at the transmitting end and a corresponding receiving antenna at the receiving end. Since channel conditions may not be all the same among the signal transmission channels, wireless network apparatus manufacturers usually use adaptive modulation coding (AMC) technology to transmit the divided signals. With AMC technology, when a signal transmission channel is in good condition, i.e., a low signal error rate or a high signal to noise ratio (SNR), the signal is transmitted in a transmission mode featuring a higher transmission speed. Conversely, the signal is transmitted in a transmission mode featuring a lower transmission speed. In this way, a better data throughput is achieved in the MIMO antenna system. Here, the data throughput is defined as the number of correct signals received by the receiving end within a unit time.
To implement the AMC technology, the conventional practice is to acquire characteristics of the individual signal transmission channels by simulation or practical measurements, and to predetermine a system threshold value related to the signal transmission channels. Then, during the operation of the MIMO antenna system, a signal transmission mode will be determined for the signal transmission channels according to the predetermined system threshold value. Table 1 (below) shows the signal transmission modes versus predetermined system threshold values in a conventional MIMO antenna system.
TABLE 1Data rateModulation scheme, Code rate(kbits/sec)Range of SNRQPSK, ½228SNR < 10 dB16QAM, ½46210 dB ≦ SNR < 32 dB64QAM, ⅔942SNR ≧ 32 dB
In table 1, the signal transmission modes (modulation scheme, code rate) are (QPSK, ½), (16 QAM, ½) and (64 QAM, ⅔), while the predetermined system threshold values are set to be 10 dB and 32 dB. In the conventional MIMO antenna system, if the receiving end measures an SNR of the signal transmitted via a first signal transmission channel to be 20 dB, which falls within an SNR range of 10 dB≦SNR<32 dB, the transmitting end may set the transmission mode of the first signal transmission channel to be (16 QAM, ½), which corresponds to a data rate of 462 kbits/sec. If an SNR of the signal transmitted via a second signal transmission channel in the MIMO antenna system is measured to be 60 dB, which falls within an SNR range of SNR≧32 dB, the transmitting end will set the transmission mode of the second signal transmission channel to be (64 QAM, ⅔), which corresponds to a data rate of 942 kbits/sec. The SNR of the signal transmitted via the second signal transmission channel is much higher than that of the signal transmitted via the first signal transmission channel, which implies that the second signal transmission channel experiences a better channel condition than the first signal transmission channel. The second signal transmission channel may use a transmission mode with a higher data rate to transmit a signal. In this way, the MIMO antenna system using AMC technology can maximize the data throughput.
However, in a realistic environment, parameters related to each signal transmission channels of an MIMO antenna system vary with time. If the related parameters of a signal transmission channel changed after a period of time due to environmental changes while the corresponding system threshold value of the signal transmission channel remains the same, the overall data throughput of the MIMO antenna system will be adversely affected because of setting the wrong transmission mode.
Accordingly, it is important to decrease the error rate of data transmissions in an MIMO antenna system when the related parameters of the signal transmission channel vary with time.